Two-dimensional, /sup 3/He neutron detectors with pad readout for high rates

We describe initial results from a new technical approach aimed at providing advanced, two-dimensional, thermal neutron detectors for high rate applications at new facilities such as the Spallation Neutron Source. The concept is based on neutron conversion in ³He. The gas mixture is contained in a grid-less ionization chamber, with the conversion depth bounded by an entrance window and an anode plane comprising discrete pixels, or pads, that are read-out in parallel. Weighting field calculations show that a ratio of gas depth to anode pad spacing of around 7:1 or greater will permit observation of almost all of the primary ionization on one pad, or a small group of adjacent pads. We illustrate with prototype detector measurements that it should be possible to construct large 1 m² devices that are capable of sustaining rates up to 10⁸ n s^-1, with position resolution of a mm or so. Operating with unity gain, this technique is extremely reliable and should provide advanced solutions to small angle scattering and reflectometry instruments